http-push: free transfer_request strbuf

http-push: free transfer_request strbuf

When we issue a PUT, we initialize and fill a strbuf embedded in the
transfer_request struct. But we never release this buffer, causing a
leak.

We can fix this by adding a strbuf_release() call to release_request().
If we stopped there, then non-PUT requests would try to release a
zero-initialized strbuf. This works OK in practice, but we should try to
follow the strbuf API more closely. So instead, we'll always initialize
the strbuf when we create the transfer_request struct.

That in turn means switching the strbuf_init() call in start_put() to a
simple strbuf_grow().

This leak is triggered in t5540.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>